1. Field of the Invention
The present invention relates to an attitude controller for a vehicle that banks while traveling through a curve, and a saddle riding type vehicle including the attitude controller.
2. Description of the Related Art
A mechanism to detect a skid motion of a wheel while a vehicle travels through a curve has been described conventionally with the use of a friction circle. The friction circle is a circle representing the maximum tolerance amount of a gripping force of a wheel. The skid motion of a wheel is described with reference to FIGS. 8 and 9. FIGS. 8 and 9 each illustrate a relationship between a force generated in the wheel and the friction circle.
FIG. 8 illustrates a relationship between a force generated in the wheel when a vehicle travels through a curve while maintaining a stable state and the friction circle. When the resultant of a longitudinal force, as the sum of a forward drive force and a backward braking force, and a cornering force generated laterally when travelling through a curve falls within the friction circle, the wheel rotates stably with no skid.
FIG. 9 illustrates a relationship between a force generated in the wheel when the vehicle travels through a curve with a skid motion and the friction circle. When the resultant of the vertical force, as the sum of the drive force and the braking force, and the cornering force generated when travelling through a curve lies outside the friction circle, the wheel skids to cause the vehicle to generate a skid motion. The skid motion of the vehicle leads to an unstable attitude of the vehicle.
An attitude controller of a four-wheeled vehicle is well-known and controls the resultant during traveling through a curve. On the other hand, a saddle riding type vehicle is banked, which differs from driving a four-wheeled vehicle. Thus, the attitude controller of a four-wheeled vehicle is not applicable to a saddle riding type vehicle. Accordingly, Japanese Patent 4402379 discloses an attitude controller specifically for a saddle riding type vehicle when travelling through a curve.
Japanese Patent 4402379 discloses an attitude controller performing a stabilizing method. In the method, output signals from a lateral acceleration sensor installed on the vehicle are evaluated to determine whether or not a braking force is to be decreased, and it is determined with which wheel the braking force is to be decreased based on output signals from a yaw rate sensor.
In the attitude controller described in Japanese Patent 4402379, the lateral acceleration sensor installed on the vehicle is inclined along with the vehicle when travelling through a curve. Consequently, accurate lateral acceleration, i.e., horizontal acceleration orthogonal to gravity, is not detectable, causing various detected values of the lateral acceleration to depend on the bank angle. Such an inconvenience may occur. FIG. 10 illustrates conventional detection of values by the lateral acceleration sensor. Here, it is assumed that a backward/forward direction of the vehicle body is an X-axis, a lateral direction of the vehicle body is a Y-axis, and a vertical direction of the vehicle body is the Z-direction. The lateral acceleration sensor detects a horizontal acceleration value when the vehicle body 62 does not bank, but detects an orthogonal acceleration value relative to the Z-axis of the vehicle body, not the horizontal value, when the vehicle body 62 banks. The value detected by the lateral acceleration sensor varies by an angle β in accordance with the bank angle when travelling through a curve. As a result, detection accuracy varies in accordance with the bank angle, and thus it becomes difficult to detect the skid motion of the wheel 61 as the bank angle increases. Such an inconvenience has occurred.